System and method for storing energy in form of compressed air in tubes integrated in a tank containing water and water vapour

ABSTRACT

The present invention relates to a system and to a method for storing energy in form of compressed air, consisting of an assembly of connected tubes forming a storage volume, the assembly being confined in a pressure-resistant thermally-insulating tank. The storage system according to the invention comprises means for storing and releasing the heat of the compressed air so as to increase the storage system efficiency.

FIELD OF THE INVENTION

The field of the present invention concerns compressed air energy storage (CAES). The invention relates to an optimized air storage system.

In this system, the energy that may come from electricity notably generated from renewable sources and that is intended for use at another time than the time of production can be stored in form of compressed air. The excess electricity produced is therefore fed into one or more compressors whose purpose is to compress a given amount of air and to store it in suitable tanks.

BACKGROUND OF THE INVENTION

To date there is no semi-massive local storage of industrial compressed air, in artificial reservoirs (unlike existing geological storages in saline cavities), between 70 and 120 bars for volumes ranging between 1000 m³ and 30,000 m³ installed on land surface and sub-surface (hereafter referred to as “mini CAES”).

Furthermore, with the current solutions, it is not always possible to store the heat energy of the compressed air, which results in energy losses of the energy storage systems.

The present invention thus concerns a system for storing energy in form of compressed air, consisting of an assembly of connected tubes forming a storage volume, the assembly being confined in a pressure-resistant thermally-insulating tank. The storage system according to the invention comprises means for storing and releasing the heat energy of the compressed air so as to increase the storage system efficiency.

SUMMARY OF THE INVENTION

The invention concerns a system for storing energy in form of compressed air, comprising at least one tube forming a storage volume, said tube being confined in a sealed pressure-resistant thermally-insulating tank. Said system for storing energy in form of compressed air includes means for storing and releasing the heat of the compressed air comprising water and/or water vapour.

According to the invention, said tube is made of metal and notably steel.

Advantageously, the water and the water vapour of said heat storage and release means occupy at least partly the inner space of the tank.

According to one embodiment of the invention, said storage system comprises an assembly of connected tubes forming the storage volume, said assembly comprising straight tubes joined in bundles and arranged in parallel.

Alternatively, said storage system comprises an assembly of connected tubes forming the storage volume, said assembly being arranged in form of a tube coil.

According to an aspect of the invention, said storage system is arranged at the surface or it is at least partially buried.

Advantageously, said tubes are arranged substantially horizontal.

According to a feature of the invention, said system comprises a secondary heat exchanger for heating said heat storage and release means.

Furthermore, said system can comprise a heat source external to said tank for heating said heat storage and release means.

According to a variant embodiment of the invention, said system comprises a tertiary heat exchanger for heating said storage volume.

Preferably, said tank consists of a closed concrete shell provided with a thermal insulating coating.

Furthermore, the invention relates to a method for storing and recovering energy by means of compressed air, wherein the following stages are carried out:

a) compressing the air,

b) storing the compressed air in a storage system as described above, and

c) expanding the stored air so as to generate energy.

According to one embodiment, the stored air is heated prior to stage c).

Advantageously, energy is generated by means of at least one turbine.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the method according to the invention will be clear from reading the description hereafter of embodiments given by way of non limitative example, with reference to the accompanying figures wherein:

FIG. 1 schematically shows a cross-sectional view of an assembly of straight steel tubes arranged in “bundles” in an insulating shell,

FIG. 2 shows an arrangement of heating and/or heat recovery pipes,

FIG. 3 illustrates an example of dimensioning storage coils according to the invention,

FIGS. 4a to 4c schematically show embodiments of vertical-axis and horizontal-axis coils and coil assembly arrangements,

FIG. 5 illustrates a storage system according to one embodiment of the invention prior to the compressed air filling stage,

FIG. 6 illustrates the storage system of the embodiment of FIG. 5 during the compressed air filling stage,

FIG. 7 illustrates the storage system of the embodiment of FIG. 5 during the compressed air withdrawal stage,

FIG. 8 illustrates a first variant of the storage system with an external heat source, during the compressed air heating stage,

FIG. 9 illustrates a second variant of the storage system with a secondary heat exchanger, during the compressed air heating stage, and

FIG. 10 illustrates a third variant of the storage system with a tertiary heat exchanger, during the compressed air heating stage.

DETAILED DESCRIPTION

According to the invention, the air under high pressure (compressed air), between 70 and 120 bars for example, is stored in at least one tube, notably made of metal, preferably steel. Steel tubes are suitable because they need to withstand the pressure of several hundred bars and have the highest possible thermal conductivity. In case of a storage using a plurality of tubes, the tubes are connected, thus forming an assembly of tubes. The assembly of tubes makes up a compressed air storage volume. This assembly of tubes is confined in a sealed pressure-resistant thermally-insulating tank. The pressure resistance allows to contain the water vapour used, as described in the description below. The thermal insulation notably allows to prevent heat losses to the outside. For example, the tank can comprise a sealed shell withstanding the inside pressure and a thermally insulating shell for limiting heat waste to the outside environment. The thermally insulating shell can be arranged at the surface or at least partially buried (i.e. half buried) so as to benefit from a more temperature stable environment and/or for esthetic, environmental and safety reasons. In the rest of the description, the expression “assembly of tubes” also relates to the embodiment where storage is achieved with a single tube.

These tubes can be protected against external corrosion by means of polyolefin, polyethylene (PE) and polypropylene (PP) coatings, and against inside corrosion by means of epoxy coatings.

According to an embodiment of the invention, metal tubes are preferably used, assembled by welding and arranged horizontally, in bundles, at the surface. The tubes, made of steel for example, are preferably arranged horizontally for greater ease of assembly over a greater length and in order to reduce the number of end plugs and of connections between tubes.

To achieve this storage system, it is possible to use preferably “seamless” oil pipes (of tubing or casing type), or straight welded tubes with a longitudinal or helical welding, used for gas pipelines, within the limits of their working pressure.

The tubes, advantageously unitary 12-15 meter elements, are preferably assembled and welded on the storage site, with limited handling means.

The tube lengths can reach several ten meters depending on the desired air volume and on the room available on the storage site. The tubes thus formed are arranged parallel to one another along their axes, then in successive layers so as to make up a “bundle” of, for example, about 10 tubes per row and about 10 tubes on top of one another, so as to obtain one hundred tubes. A bundle is understood to be a pile of several rows of tubes, each row comprising a plurality of tubes.

The tubes are connected to one another by pipes of suitable diameter enabling to communicate them, in parallel or in series, so as to obtain the desired storage volume. A set of valves on these pipes allows a tube or an assembly of tubes to be isolated.

The “bundle” formed is preferably quasi-horizontal, with however a slight slope to facilitate the flow of condensates.

The tubes are preferably arranged horizontal rather than vertical. Horizontally, they allow to obtain a greater storage volume with a minimum number of connecting pipes and tube end plugs, unlike the vertical half-buried arrangement which involves that the tube length is limited to the excavation depth. The multiplication of vertical tubes increases the number of end plugs, complicates connections and increases the cost of the plant.

Example of Orders of Magnitude:

For a steel tube of 20″ (0.508 m) outside diameter, 0.485 m inside diameter and 10 m in length, and for a bundle of 10×10=100 tubes, the weight is 137 tons for a compressed air volume of 185 m³, which gives a m³ of air/tons of steel ratio of 1.348, or the ratio of 0.742 tons of steel/m³ stored. This calculation does not take account of the tube end plugs and the connecting means between them. Therefore, the rule recommending “one kilo of steel per liter of compressed air stored” can be applied.

In a variant according to the invention, if the stored air is hot (heat from the compression stage), the goal is to keep as long as possible the heat of the compressed air in the storage tubes. The tank containing the assembled straight tube bundles is therefore thermally insulated from the outside environment so as to keep the heat of the compressed air fed into the storage. FIG. 1 is a cross-sectional view of a bundle made up of parallel tubes 1 confined in an enclosure 2 (i.e. an insulated pressure-resistant tank) for thermal insulation, environmental and/or safety reasons.

Additional heating of the air contained in horizontal straight tubes can be achieved internally or externally of the storage tube.

Besides, it is possible to arrange in each storage tube a tube referred to as “insert”, filled with a heat storage material.

Another variant consists in using a heat exchanger made up of pipes 3 arranged in the space available between the compressed air storage tubes. The tubes are preferably in metal-metal contact so as to facilitate heat transfer by conduction. FIG. 2 shows another arrangement for the parallel tubes and heat exchange pipes 3.

Heating or maintaining the temperature of the compressed air contained in the storage tube is achieved through circulation of a hot fluid (vapour or a heat-carrying liquid) that exchanges, by thermal conductivity of the steel between the two types of tube (compressed air tubes and hot fluid pipe), heat with the stored compressed air.

Another embodiment of the compressed gas storage consists in forming steel tube coils. The coils are preferably, but not necessarily, formed on the site due to their size: dimensions of the coils, weight and of the handling means available.

Manufacture of the unitary coil can be achieved by winding the steel tube, through plastic deformation thereof, in successive layers around a drum of horizontal or vertical axis. FIG. 3 shows a section along a diameter of a coil.

Preferably, the axis of the coil is vertical.

Example of Orders of Magnitude:

A unitary storage coil of 15 m maximum diameter, 3 m minimum diameter, 1.5 m in width, with a steel tube of 6.625″ (16 cm) diameter, 6.4 mm thickness, has a steel weight of 230 tons, and a gas storage volume of 176 m³. The volume of the cylinder made up of such an elementary coil is 276 m³.

FIGS. 4b and 4c schematically show a horizontal-axis coil 6 and an assembly of vertical-axis coils 7.

As illustrated in FIG. 4a , the coils can be made from standard steel tube sections of 12-15 m unitary length, assembled and welded so as to form a continuous tube that is subsequently subjected to plastic bending by means of pinch rolls 8 and wound on a specific drum 9 so as to form an elementary coil, with a volume of 276 m³ for example.

Several unitary coils can be superposed and interconnected by pipes so as to make up a storage volume in a tank (FIG. 4c ).

The dimension of the manufactured coil and therefore the weight thereof directly depend on the lifting capacity on the mini CAES site. Each coil element can be manufactured on site, hooped and turned over to switch from a vertical coil (with its horizontal axis) to a horizontal coil (with its vertical axis), then positioned, lowered and laid flat in the bottom of previously built tank 10 (FIG. 4).

An excavation can be performed to contain the lower half of the drum, in order to avoid an overhead structure and to facilitate winding of the continuous tube.

A gantry compatible with the loads and dimensions of the coil (for example 15 m in outside diameter, 3 m in height and a coil weight of 230 tons) allows the coil to be tipped over, moved and lowered into the tank.

It will be possible to manufacture coil elements of smaller height (2 m for example) while keeping an acceptable outside diameter (15 m for example) so as to maintain the linear volume and to reduce the mass of the manufactured element.

The coil elements can thus be stacked on one another and connected by means of tubes, arranged in series or in parallel.

The compressed air storage tubes can be, upon construction of the coil, joined (contiguous spires) or spaced a few centimeters apart between each tube, so as to facilitate heat exchanges between the steel of the tube and the heat-carrying fluid if the air is stored hot.

According to the invention, in order to store the heat of the stored compressed air, the energy storage system comprises means for storing and releasing the heat of the compressed air. The heat storage and release means allow to store the heat of the compressed air during the storage stage because the compressed air is heated by the compression thereof. The heat storage and release means allows to release the heat stored upon withdrawal of the compressed air because the compressed air is cooled by the expansion thereof at the outlet of the hot compressed air storage system. This expansion of the hot stored compressed air can be achieved by means of an expansion turbine or any other system, with pistons for example, allowing to convert the stored energy to mechanical rotational or even electrical energy if the expansion means are coupled with an electric generator. Thus, the energy efficiency of the storage system is increased.

The heat storage and release means comprise water, in liquid and gas (water vapour) form, for storing the heat. During the storage stage, the water in liquid form vaporizes and, during the release stage, the water in vapour form condenses. Preferably, the water and the water vapour occupy at least partially the inner space of tank 2 provided with a thermally insulating shell. Advantageously, the water and the water vapour occupy the space between the tubes forming the compressed air storage volume. The heat transfer between the compressed air and the water vapour occurs directly, the thermal conduction of the material (metal for example) forming the tubes. The heat energy (or heat) stored depends on the sensible heat of the water (energy of the water at saturation temperature) increased by the latent heat (water vaporization energy). In the rest of the description and in the figures, the term “water” designates water in liquid form and the term “vapour” designates water in water vapour form.

Using water and water vapour allows to obtain a good thermal efficiency for the energy storage. Indeed, water vapour has physical properties that are particularly well suited for heat transfer; in particular, the latent heat thereof allows to obtain a good thermal efficiency. Furthermore, heat storage and release cycles using water vapour can be rapid. Besides, water vapour can be used for a larger number of cycles than heat absorbing materials that degrade over time. Moreover, water affords the advantage of being chemically stable and inexpensive.

The tank is designed to withstand the water vapour pressure. For example, the tank can preferably be made of concrete internally and/or externally coated with a thermally insulating material. Thus, the tank can withstand high inside pressures (water vapour pressure) of about 10 bars for example and high temperatures (water vapour temperature) of about 180° C. for example. Alternatively, the sealed pressure-resistant tank can also be made of sheet steel or sheet steel externally reinforced by a concrete cover.

FIGS. 5 to 7 illustrate the same embodiment of the storage system according to the invention, respectively prior to compressed air filling of tubes 1 (FIG. 5), during compressed air filling of tubes 1 (FIG. 6) and during compressed air release (FIG. 7).

As illustrated in FIG. 5, tank 2 with its thermally insulating shell is initially partly filled with water, cold water for example, prior to filling tubes 1 with hot compressed air. The water occupies the lower part of the inner space of tank 2 between the assemblies of tubes 1, up to a height H. Tank 2 comprises in the lower part thereof compressed air delivery means 11 that can be connected (or provided) to a compression unit (or a compressor) and to at least one tube 1 of the assembly. Besides, tank 2 (the thermally insulating shell) comprises in the upper part thereof compressed air discharge means 12 that can be connected to an expansion system (provided with an expansion valve) and to at least one tube 1 of the assembly. According to a variant embodiment that is not shown, the compressed air expansion system that converts the hot air compression energy to mechanical rotational energy can be an assembly made up of turbines or piston engines. The rotational energy produced can be converted to electrical energy by an electric generator.

According to the invention, the hot compressed air at the outlet of a compressor, at 400° C. for example, is stored in the storage volume made up of tubes 1. The heat or the heat energy entering tank 2 is provided by the hot compressed air through metal tubes 1. According to a variant embodiment of the invention, the compressor(s) is (are) arranged as close as possible to tank 2 so as to limit thermal losses in the tubes outside the tank. As illustrated in FIG. 6, the lower part of tank 2 is initially filled with water in which tubes 1 of the assembly are partially immersed. Due to the heat supplied, the water progressively vaporizes until a water-vapour medium in phase equilibrium depending on the pressure and temperature of the medium is obtained; thus, the temperature and the pressure of the water and vapour phases increase. In tank 2, the vapour volume increases and the water volume decreases (water height H decreases) until complete thermal energy charge of tank 2 that is limited by the maximum vapour pressure allowed by storage tank 2.

During the stage of withdrawing compressed air from the tubes, according to the ideal gas law (of Amontons, who specifies that, “for a constant volume of a gas, the pressure varies when the temperature changes”), decompression of the air in the constant volume of the tubes produces a decrease in the temperature of the stored air. The air temperature decrease is compensated by the heat supplied by the vapour under pressure and the previously stored water. The vapour yields the latent heat thereof and possibly a small part of the sensible heat of the water. The condensates return to the tank as water; the water volume in the tank does therefore not vary during the vaporization-condensation cycle since the vapour is not vented to atmosphere. As illustrated in FIG. 7, during the compressed air withdrawal stage, water height H increases in tank 2.

According to a variant embodiment of the invention, the energy storage system (in form of hot compressed air) can comprise an external heat source for increasing the heat storage in the tank and for compensating, notably in case of long-term storage, the losses due to imperfect insulation. For example, the energy storage system can comprise a vapour accumulator arranged outside the compressed air storage system. Injecting vapour at sufficient pressure and temperature into the tank increases the heat storage in the tank. For this variant embodiment, the volume of water in the tank at the end of the compressed air withdrawal stage has increased in relation to the water volume initially present in the tank. This water volume depends on the amount of vapour injected. This water vapour reserve can be waste vapour or water vapour created from the combustion of fossil energy (by means of a burner for example), “stray” heat from the industry (refining, iron and steel industry, etc.), household waste incinerators, or intermittent energy (wind or solar energy for example). The tank accumulating the heat from an external source can be arranged at the surface or at least partially buried.

FIG. 8 illustrates an example of this variant embodiment during the compressed air storage stage. The inner space of tank 2 is connected to a heat accumulator tank 13. Tank 13 comprises water and water vapour. The water vapour of tank 13 is directly injected into the water of tank 2, preferably at the base thereof, which is filled with water. Heat accumulator tank 13 allows the storage of heat in tank 2 to be increased. FIG. 8 also illustrates the compressed air delivery 11 and discharge 12 means.

According to a variant embodiment of the invention, the energy storage system can comprise an exchanger for heat exchange between the water contained in tank 2 and a heat-carrying fluid. This heat exchanger is referred to as secondary heat exchanger. The secondary heat exchanger is arranged in the tank, mainly in the lower part thereof. It is at least partially immersed in the water. This secondary heat exchanger can consist of an assembly of tubes where the liquid heat-carrying fluid circulates. The secondary heat exchanger allows the water to be heated, which allows the heat storage in the tank to be increased.

FIG. 9 illustrates an example of this variant embodiment during the compressed air storage stage. The lower part of tank 2, free of tubes 1, comprises a secondary heat exchanger 14. A hot heat-carrying fluid (preferably always in the liquid phase) circulates in secondary heat exchanger 14 through an inlet 16 and an outlet 17, and it heats the water contained in tank 2. FIG. 9 also illustrates the compressed air delivery 11 and discharge 12 means.

According to a variant embodiment of the invention, the energy storage system can comprise an exchanger for heat exchange between the compressed air and a heat-carrying fluid. This heat exchanger allows to heat the compressed air storage volume, i.e. the tubes. This heat exchanger is referred to as tertiary heat exchanger. The tertiary heat exchanger can be internal to the tubes and it can come in form of “inserts” within the tubes in which the heat-carrying fluid circulates. This tertiary exchanger allows the compressed air to be directly heated, which allows the thermal efficiency of the energy storage to be increased.

FIG. 10 illustrates an example of this variant embodiment. For figure readability reasons, FIG. 10 is represented in a direction perpendicular to the direction of FIGS. 5 to 9. Furthermore, only two tubes 1 are shown. An insert is arranged within tubes 1. The insert comprises a tertiary heat exchanger 15 in which a hot heat-carrying fluid that directly heats the compressed air stored in tubes 1 circulates. FIG. 10 also illustrates the compressed air delivery 11 and discharge 12 means.

Besides, according to a variant embodiment, the energy storage system can comprise a tertiary heat exchanger external to the tubes for heating the stored compressed air. FIGS. 1 and 2 illustrate such heat exchangers 3.

According to another variant embodiment that is not shown, a burner can also be provided as the external heat source.

FIGS. 5 to 10 illustrate by way of non limitative example several embodiments of the storage system according to the invention in cases where the assembly of tubes comprises straight tubes arranged in bundles. However, these embodiments are also suited in the case of tube assemblies arranged in form of coils, as illustrated in FIGS. 3 and 4.

The various embodiments can be combined. For example, the energy storage system can comprise both an external heat source, a secondary heat exchanger and a tertiary heat exchanger.

The invention furthermore relates to an energy storage and recovery method using compressed air. The method implements a storage system as described above. During the process, the following stages are carried out:

-   -   compressing air, notably using compression means,     -   storing the compressed air in the tubes of the storage system as         described above,     -   expanding the stored compressed air contained in the tubes so as         to generate energy, notably mechanical or electrical energy, by         means of a turbine.

The method can also involve a stage of heating the stored compressed air by means of the storage systems according to the embodiments of FIGS. 8 to 10. 

1. A system for storing energy in form of compressed air, comprising at least one tube forming a storage volume, said tube being confined in a sealed pressure-resistant thermally-insulating tank, characterized in that said system for storing energy in form of compressed air includes means for storing and releasing the heat of the compressed air comprising water and/or water vapour.
 2. A system as claimed in claim 1, wherein said tube is made of metal and notably steel.
 3. A system as claimed in claim 1, wherein the water and the water vapour of said heat storage and release means occupy at least partly the inner space of tank.
 4. A system as claimed in claim 1, wherein said storage system comprises an assembly of connected tubes forming the storage volume, said assembly comprising straight tubes joined in bundles and arranged in parallel.
 5. A system as claimed in claim 1, wherein said storage system comprises an assembly of connected tubes forming the storage volume, said assembly being arranged in form of a tube coil.
 6. A system as claimed in claim 1, wherein said storage system is arranged at the surface or it is at least partially buried.
 7. A system as claimed in claim 1, wherein said tubes are arranged substantially horizontal.
 8. A system as claimed in claim 1, wherein said system comprises a secondary heat exchanger for heating said heat storage and release means.
 9. A system as claimed in claim 1, wherein said system comprises a heat source external to said tank for heating said heat storage and release means.
 10. A system as claimed in claim 1, wherein said system comprises a tertiary heat exchanger for heating said storage volume.
 11. A system as claimed in claim 1, wherein said tank consists of a closed concrete shell provided with a thermal insulating coating.
 12. A method for storing and recovering energy by means of compressed air, wherein the following stages are carried out: a) compressing the air, b) storing the compressed air in a storage system as claimed in claim 1, and c) expanding the stored air so as to generate energy.
 13. A method as claimed in claim 12, wherein the stored air is heated prior to stage c).
 14. A method as claimed in any one of claim 12, wherein energy is generated by means of at least one turbine. 